Viruses initiate infection by attaching to specific cell surface receptors. This interaction is critical for a successful infection and is invariably an important determinant of viral pathogenesis. One goal of this proposal is to complete our effort to clone a mouse gene, Rmc-1, that confers infectability for the MCF class of C-type retroviruses and is thought to encode a cell surface receptor for these viruses. if as predicted this gene encodes an MCF virus receptor, we wish to determine whether this gene plays a role in the in vivo tropism and leukemogenicity of MCF viruses, and study structure-function relationships by analyzing naturally occurring Rmc-1 variants that confer altered host range. We are using physical mapping techniques combined with transfection assays to clone Rmc-1 and have made significant progress towards our goal. A second goal of this proposal is to develop retroviral vector (VSV-G) pseudotypes, including gene trap vectors, as insertional mutagens in zebrafish. These pseudotypes have already been shown to infect cultured zebrafish cells. We will determine the best method to infect the zebrafish germ line and determine the frequency with which infection disrupts genes to cause mutations. We will then perform an insertional mutagenesis screen to identify genes that play key roles in early zebrafish development. If successful, these studies would have a major impact on the genetic analysis of vertebrates including studies of genetic and developmental dise e.